1. Field of the Invention
High temperature alloys for thermal equipment based on intermetallic compounds which are suitable for ordered solidification and supplement the conventional nickel-based superalloys.
The invention relates to the further development and improvement of the alloys based on an intermetallic compound of the titanium aluminide TiAl type with further additives which increase the strength, the toughness and the ductility.
In the narrower sense, the invention relates to a high temperature alloy for machine components based on doped TiAl.
2. Discussion of Background
Intermetallic compounds of titanium with aluminum have some valuable properties which make them appear attractive as structural materials in the medium and higher temperature range. These include, inter alia, their density, which is low compared with superalloys and reaches only about half the value for Ni superalloys. However, their brittleness stands in the way of their industrial applicability in the present form. The former can be improved by additives, in which case higher strength values are also achieved. Possible intermetallic compounds, some of which have already been introduced, which are known as structural materials are, inter alia, nickel aluminides, nickel silicides and titanium aluminides.
Attempts have already been made to improve the properties of pure TiAl by slight modifications of the Ti/Al atomic ratio and by alloying with other elements. Further elements proposed were, for example, alternatively Cr, B, V, Si, Ta as well as (Ni+Si) and (Ni+Si+B), and also Mn, W, Mo, Nb, Hf. The intention was, on the one hand, to reduce the brittleness, that is to say to increase the ductility and toughness of the material, and, on the other hand, to achieve as high a strength as possible in the temperature range of interest between room temperature and operating temperature. An additional aim was a sufficiently high resistance to oxidation. These aims were, however, only partially achieved.
The high temperature strength of the known aluminides in the meantime still leaves something to be desired. Corresponding to the comparatively low melting point of these materials, the strength, in particular the creep resistance in the upper temperature range, is inadequate, as can also be seen from relevant publications.
U.S. Pat. No. 3,203,794 discloses a TiAl high temperature alloy containing 37% by weight of Al, 1% by weight of Zr and remainder Ti. The comparatively small addition of Zr causes this alloy to have properties comparable to those of pure TiAl.
EP-A1-0,365,598 discloses a high temperature alloy based on TiAl with Si and Nb additives, whereas in EP-A1-0 405 134 a high temperature alloy based on TiAl with Si and Cr additives is proposed.
The following documents are also cited in respect of the prior art:
N. S. Stoloff, "Ordered alloys-physical metallurgy and structural applications", International metals review, Vol. 29, No. 3, 1984, pp. 123-135. PA0 G. Sauthoff, "Intermetallische Phasen" ("Intermetallic Phases"), Werkstoffe zwischen Metall und Keramik, Magazin neue Werkstoffe January, 1989 p. 15-19. PA0 Young-Won Kim, "Intermetallic Alloys based on Gamma Titanium Aluminide", JOM, July 1989. PA0 U.S. Pat. No. 4,842,819 PA0 U.S. Pat. No. 4,842,819 PA0 U.S. Pat. No. 4,842,820 PA0 U.S. Pat. No. 4,857,268 PA0 U.S. Pat. No. 4,836,983 PA0 EP-A-0,275,391
The properties of the known modified intermetallic compounds in general do not yet meet the technical demands for the production of usable workpieces therefrom. This applies in particular with regard to high-temperature strength and toughness (ductility). There is therefore a need for further development and improvement of such materials.